1. Field of the Invention
The present invention relates to an antenna system for satellite communication.
2. Description of the Related Technology
Maritime satellite communications were started in 1976 using the Marisat system. In 1982, Marisat was handed over to the internationally organized Inmarsat system and has been in operation since then. Since 1982, the United States military has placed in orbit several satellite systems used for communication.
The Ultra High Frequency Satellite Communications (UHF SATCOM) system provides communication links, via satellite, between designated mobile units and shore sites worldwide. The UHF SATCOM system is one of three SATCOM systems installed, and operates in the UHF range. The SATCOM systems, combined, represent a composite of information exchange systems that use the satellites as relays for communications and control as well as quality monitoring subsystems that provide data to manage satellite resources. UHF SATCOM generally involves vehicle earth station antennas having a desired amount of gain. Ground vehicles on-the-move (OTM) have usually been limited to low data rates (voice) and use “omni” antennas that use circular polarization and are pointed straight up. For higher gain, high data rate communications to low elevation satellites, where the omni antennas have too low gain, man-portable high gain adjustable pointing antennas have been used at halt. To achieve high data rate OTM communications to low elevation satellites, high gain antennas must usually be used and these high gain antennas must usually be pointed dynamically as the vehicle maneuvers. As the operators are usually very busy and are used to the near-zero attention required by the SATCOM omnis and antennas of their other communications systems, a steerable antenna should also help to minimize workload on the operator.
Directional antennas, such as parabolic reflector antennas or yagi type antennas, are commonly housed under domes. A radome is usually necessary to make the antenna resistant to object collisions. The radome is usually mounted on a radome base and is removable to facilitate maintenance and repair work. Such radomes may be easily identifiable on a vehicle and may attract unwanted attention.
Highly directional antenna systems installed on a moving platform are usually steerable so as to adequately receive radio waves from remote satellites. To continuously point in the direction of the satellite under platform heading motions, the antenna is commonly steered by mechanical or electronical means. A variety of technologies have been developed to steer the antenna to point at the satellite under heading changes.
Global positioning system (GPS) receivers are commonly used in such steering systems to provide a moving platform's track, wherein the track is the vehicle's course over ground. Generally, a land-based vehicle's track will be coincident with its heading. However, there are several situations in which the vehicle's, heading is not adequately provided by the GPS receiver. These situations include: inadequate GPS reception, low speeds where the GPS track is inherently noisy, and vehicles backing up. In one example, a moving vehicle which has been parked, in which the steering system has been turned off, and which has subsequently shifted direction before turning the steering system back on will have a heading that is different than its track. When the vehicle is turned back on, the vehicle will be pointing in a direction that is different than the last known track of the GPS receiver. An antenna steering system which can assess the vehicle's heading in all situations is needed.
Antenna systems installed on moving platforms also need to be malleable to obstacles such as, for example, tree branches, bridges with low clearance, etc.